Attenuated total (internal) reflection (ATR) spectroscopy is widely used to collect an absorption spectrum from samples that are too opaque for direct absorption measurements. One surface of an ATR crystal is placed in contact with a sample-under-test. An incident beam of radiation is directed through the ATR crystal so that it is totally internally reflected at the boundary between the ATR crystal and the sample-under-test. Some of the energy of the incident radiation is absorbed by the sample-under-test through evanescent coupling. The amount of absorption is representative of the molecular structure and/or the molecular species found in the sample-under-test. The reflected radiation, therefore, includes information from which an absorption spectrum for the sample-under-test is obtained.
Examples of systems for performing ATR spectroscopy include U.S. Pat. Nos. 3,393,603, 4,602,869, and 5,093,580. These systems are limited in that they typically employ a single-element detector and, therefore, are able to analyze only a small area of the sample-under-test at one time. Accordingly, some type of physical scanning is required to resolve the spatial distribution of molecular species in the sample by, for example, translating the sample on an XY stage through a field of view defined by the collecting and detecting optics, or translating the ATR relative to the sample.
Scanning the sample in this manner involves certain drawbacks and deficiencies. Specifically, the number of moving parts required to implement the scanning limits the speed and reliability of such systems. Furthermore, the signal to noise ratio obtainable by most FTIR (Fourier transform infrared) spectrometers requires that several measurements be taken at each point and averaged. The substantial averaging required makes such systems inherently slow.